The present invention relates to a process for the regeneration of spent catalyst from a fluid catalytic cracking unit (FCCU). More particularly, the invention relates to the recovery of power from hot regeneration gases recovered from such regeneration process.
In a typical FCCU, spent catalyst is continuously removed from the reactor, sent to the regenerator, and then returned to the reactor. In the regenerator, the fouled catalyst is contacted with an oxidative regeneration gas, at elevated temperatures and pressures, to remove coke or other carbonaceous deposits from the catalyst by combustion.
Such combustion of carbonaceous deposits can be effected in a fluidization chamber containing the solid catalyst particles through which a fluidizing gas is passed upwards at a rate to maintain the particles as a fluidized bed, i.e. in a turbulent state with quasi liquid properties, including a recognizable upper level. Fluidization distributes the fluidized catalyst into a lower dense phase and an upper dilute phase. Typically, the fluidizing gas is, or at least contains, the oxidative regeneration gas. The combustion or regeneration gases produced by the burning of the carbonaceous deposits are typically at high temperatures and elevated pressures. For example, it is not uncommon for regeneration gases to have a temperature in excess of 1000.degree. F. and range up to 1500.degree. F., or even higher, while pressures may range from about 10 psig up to about 35 psig and greater. Thus the gases, commonly referred to as flue gas, emerging from the regeneration zone represent a large energy potential which may be utilized to recoup a part of the power expended in the system in compressing the air used as the oxidative regeneration gas. In some cases, enough energy is released in the regeneration process that, if properly recovered, a net gain may be realized in the regeneration, thus supplying a surplus of power for utilization in other operations, e.g. generation of electric power.
It is common practice to utilize expansion turbines or turbo expanders to recover energy from hot flue gases from regenerators. In the usual case, the flue gas, at high temperature and elevated pressure, is passed to an expansion turbine which then supplies shaft power to an air compressor used to generate compressed air for the regeneration process. Shaft power in excess of air compressor requirements is used to drive a motor-generator capable of generating electricity.
There have recently been developed fluid catalytic cracking catalysts which allow essentially complete combustion of the carbonaceous material on the spent catalyst to carbon dioxide in the dense phase zone of the regenerator, essentially no carbon monoxide being produced. The use of such catalyst is highly desirable as it prevents undesirable afterburn in the regenerator dilute phase zone, a condition brought on by the presence of both carbon monoxide and regeneration gas in the dilute phase. Additionally with the use of these catalysts, all heat of combustion may be utilized with the fluid catalytic cracking process rather than being lost completely or recovered externally of the regenerator in a carbon monoxide boiler. The latter process is disclosed, for example, in U.S. Pat. Nos. 3,137,133 and 3,139,726. The use of complete combustion catalysts, which allow increased recovery of heat of combustion, provides a higher regenerative dense phase zone temperature and thereby allows lower catalyst to oil ratios in the cracking zone and hence improved yields. As noted, the hot flue gases from the regeneration zone are typically expanded through the expansion turbine of an expander turbine-compressor set to recover energy from the flue gas. While centrifugal flow compressors or turbo blowers may be used in such a power recovery set, axial flow compressors, because of their high efficiency and higher capacity, offer certain advantages. There is, however, a distinct problem with the use of axial flow compressors in such systems. Because of the relatively steep head-capacity-characteristics of axial flow compressors, the surge point may be close to, i.e. within 10% to 30% of the design flow. This characteristic of axial flow compressors makes them susceptible to surging, i.e. unless the axial flow compressor is operated under conditions where it is required to compress more air than needed on the discharge side, the compressor will begin to surge. There is a minimum capacity for axial flow blowers below which operation becomes unstable, i.e. surging occurs. Surging results when the line pressure on the exhaust side of the compressor exceeds the exhaust pressure which the machine is capable of producing. Since the compressed gas cannot get into the outlet or exhaust line, it rushes back into the compressor. This lowers the outlet line pressure momentarily and the compressor begins to, once again, discharge into the outlet line. However, the pressure immediately gets too high in the outlet line and, again, the air cannot be discharged from the compressor. The gas then rushes back into the compressor and the entire cycle is repeated. Continued operation of a compressor under surging conditions will eventually cause the compressor to tear itself apart.
Surging in axial flow compressors can become a serious problem in fluid catalytic cracking (FCC) processes because at times, it is desirable to operate the unit under turndown conditions. In turndown conditions, the amount of feed stock to the FCCU is reduced to below design capacity which results in a low flow of the spent catalyst to the regenerator through line 10. This means that the amount of catalyst being regenerated, and accordingly the amount of compressed regeneration gas (air) being used in the regenerator and resultant hot flue gas produced is reduced. It also means that less compressor capacity is required. It has been customary practice in turndown and when using axial compressors, to require the compressor to compress more gas than needed for the regeneration process and vent the excess at the blower discharge via discharge valve 34 and line 32. While this practice prevents surging of the compressor, it represents a loss of recoverable energy in the system. Likewise, this venting of the excess blower discharge prevents excessive afterburning of CO in the dilute phase zone of the regenerator and the resultant excessive heating in the dilute phase zone when not operating under the high temperature regeneration mode.